Why doesn't apt update fetch the Contents-* files?

I set up a local APT repository, which is working fine, except that apt update doesn't fetch Contents-amd64.gz files, like it does from an official online Debian repo. [Note : apt-file is installed, and apt update from an online Debian repo does indeed fetch Contents-* files. So there *must* be a problem with my local repo.] For the purpose of this post, I stripped my local repo down to a bare minimum, with a single package in it :

$ tree /tmp/test/
/tmp/test/
├── dists
│   ├── bookworm
│   │   ├── main
│   │   │   ├── binary-amd64
│   │   │   │   └── Packages
│   │   │   │   ├── Packages.gz
│   │   │   └── Contents-amd64.gz
│   │   └── Release
│   └── stable -> bookworm/
└── pool
    └── main
        └── abcde_2.9.3-1_all.deb

My sources.list file :

deb [trusted=yes] file:/tmp/test/ bookworm main

apt update fetches correctly Release and Packages.gz (so my repo is functional), but it doesn't even try to fetch Contents-amd64.gz :

$ sudo apt update 
Get:1 file:/tmp/test bookworm InRelease
Ign:1 file:/tmp/test bookworm InRelease
Get:2 file:/tmp/test bookworm Release [1 913 B]
Get:2 file:/tmp/test bookworm Release [1 913 B]
Get:3 file:/tmp/test bookworm Release.gpg
Ign:3 file:/tmp/test bookworm Release.gpg
Get:4 file:/tmp/test bookworm/main amd64 Packages [1 039 B]
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
All packages are up to date.

Yet Contents-amd64.gz seems to be correctly referenced in the Release file :

$ cat dists/bookworm/Release 
(...)
MD5Sum:
 (...)
 2b43(...)8733    236 main/Contents-amd64.gz
SHA1:
 (...)
 9464(...)2e9d    236 main/Contents-amd64.gz
SHA256:
 (...)
 f48f(...)4135    236 main/Contents-amd64.gz
SHA512:
 (...)
 5253(...)fa43    236 main/Contents-amd64.gz

Any idea why it doesn't get fetched ? What did I miss when setting up my local repo ?